Constant force electromechanical actuator



A. w. VINCENT 3,

CONSTANT FORCE ELECTROMECHANICAL ACTUATOR Feb. 11, 1969 Filed Feb. 6, 1967 Sheet of 2 FIG. 1

28 MAGNETIC FORCE GAP LENGTH 26 CGUPLING RATIO FIG. 2

FIG. 6

INVENTOR. ANDREW W. VINCENT ATTO RN EY Feb. 11, 1969 A. w. vmcENT 3,427,576

CONSTANT FORCE ELECTROMECHANICAL ACTUATOR Filed Feb. 6, 1967 Sheet 2 of 2 FIG. 3

FIG. 5

INVENTOR. ANDREW w VINCENT ATTORNEY United States Patent 3,427,576 CONSTANT FORCE ELECTROMECHANICAL ACTUATOR Andrew W. Vincent, 65 Aberdeen St., Rochester, N.Y. 14619 Filed Feb. 6, 1967, Ser. No. 614,274 US. Cl. 335276 Int. Cl. H01f 3/00, 7/08 8 Claims ABSTRACT OF THE DISCLOSURE Brief summary of the invention This invention relates to a novel electromechanical actuator, and, more particularly, to a novel actuator of this type, which produces a relatively constant force through its entire travel, and which may readily be made to operate with a high degree of efiiciency and at high speed.

It is well known that the output forces of electromechanical actuators conform generally to a modified parabolic function of the motion of the armature toward the core. At the beginning of the stroke, the magnetic gap is at a maximum, the flux produced by the energizing current is at a minimum, and the output force is also at a minimum. As the armature moves toward the core and the gap becomes smaller, the flux and the output force both increase at increasing rates. The force exerted by the actuator at the end of its stroke is usually several times the force available at the start. This characteristic is well suited for many purposes such as for actuating multicontact electrical relays, but is very poorly suited for many other purposes. Many situations are encountered wherein it is desired to provide a relatively uniform force over the entire travel of the actuator. This has heretofore been accomplished by an actuator marketed under the trade name Ledex, in which travel of the armature toward the core is restrained by an arrangement of balls and earns, which causes the armature to rotate as it moves toward the core. Rotary actuators of this type, however, are subject to certain limitations, primarily in respect of their operating speed. The armature, which must be relatively massive to carry the necessary flux, must be accelerated through a relatively long angular displacement, and, because of the circular arrangement, it is not feasible to make either the armature or the core of laminated construction. These are among the fatcors that limit the operating speed of such actuators.

The actuator of the present invention overcomes the disadvantages of the uniform output actuators of the prior art, and enables the achievement of a significant improvement in operating speed. Laminated cores and armatures of standard, simple and highly efficient shapes may be used, and all parameters may be designed for maximum magnetic efiiciency. Frictional losses also are minimized, and a high degree of uniformity of output force is achieved. Moreover, adjustment means are provided whereby uniformity of output force may be readily maximized for each actuator during final assembly.

Briefly, the invention contemplates coupling the armature to the load through a lever carried on a floating 3,427,576 Patented Feb. 11, 1969 pivot, that is, a pivot which is freely movable in a direction normal to the axis of rotation defined by it. The effective lever arm between the pivot and the load is arranged to remain substantially uniform throughout the full motion of the actuator. The effective lever arm between the pivot and the armature, however, is caused to decrease as the armature closes. The decrease is controlled closely to match and to compensate for the increase in force developed by the armature, so that the output force is highly uniform through the entire motion. This result is achieved by the use of a cylindrically curved bearing surface to define the contact between the armature and the lever. The curvature is in a plane normal to the pivot axis. As the armature closes, the line of contact rolls along the curved bearing surface toward the pivot axis, thus shortening the lever arm between the pivot and the armature. Since the pivot floats, there is no sliding, and, consequently, almost no frictional loss.

The curvature of the bearing surface determines the nature of the variation in the effective lever arm. In the embediments shown and described herein, the bearing surface is curved along the arc of a circle, but it will be seen that the nature of the variation may be changed by giving the bearing surface a spiral curvature. In some cases, a more exact compensation may be achieved in this way for the magnetic force characteristic.

Detailed description of the invention Representative embodiments of the invention will now be described in connection with the accompanying drawings, wherein:

FIGURE 1 is a schematic diagram illustrating the geometrical principles of the invention;

FIGURE 2 is a chart showing a typical magnetic force curve for electromechanical actuators and a curve showing the leverage variation achieved in the practice of the invention;

FIGURE 3 is a side elevational view, partly in section, of an actuator of the presently preferred embodiment of the invention;

FIGURE 4 is the plan view of the actuator shown in FIGURE 3;

FIGURE 5 is an end elevational view of the actuator; and

FIGURE 6 is a fragmentary, longitudinal sectional view of an actuator according to a modified form of the invention.

The theory of operation of the actuator of the presently preferred embodiment of the invention will now be described with reference to FIGURES 1 and 2. FIGURE 1 is a geometrical diagram illustrating the derivation of the relationship between the motion of the armature and the decrease of the effective lever arm between the pivot axis and the line of contact between the armature and the pivot. The circular are 10 represents the curved bearing surface, which in the embodiment referred to is carried by the lever. The armature is represented by the simple block 12. It is always tangent to the are 10 and always horizontal as viewed in the drawing. The pivot 14 is at the point of intersection between an arbitrarily chosen radius 16 and the are 10. The angle B is the angle between the radius 16 and the radius 18 that extends to the point of tangency of the armature 12. The length of the effective lever arm is represented by the normal line 22 from the pivot 14 to the vertical radius 18. The travel of the armature 12 is through the distance if between the armature and the lever arm 22.

It should be noted that in operation, the bearing surface of the armature 12 and the effective lever arm 22 always remain parallel to each other. The pivot 14 moves around the circle, or, more precisely, the circle rolls along the armature 14.

arm 22 is /2tt and the leverage available to the armature as a function of its closing travel is VZt-t This latter is a relatively complex function. It is plotted over a limited range (from about t=.005 times the radius to about t=.07 times the radius) as the curve 26 in FIGURE 2. The typical, empirically derived magnetic force curve 28 is also shown in FIGURE 2. It too is a relatively complex curve. The two curves do not lend themselves to mathemathical comparison because the magnetic force curve 28 is empirical and does not conform exactly to any known mathematical expression. However, it is apparent that the two curves are generally similar in shape, and that it is possible to select a portion of the leverage curve 26 that approximates the magnetic force curve. In practice, many factors affect the design of an actuator according to the invention such as, for example, the efliciency of the magnetic circuit, and the amount of rotation it is desired to produce at the output. Also, for efficiency and long life, the lever should be pivoted for rocking, or rolling motion upon a knife edge, and this will limit the amount of rotation available and also the sharpness of curvature of the curved bearing surface.

It has been found that as a practical matter a satisfactory match is achievable by adjusting the position of the pivot 14 relative to the magnetic gap so that when the gap is fully closed, the angle B is about one-eighth to about one-tenth of its value at the fully open position of the gap. The optimum match is achieved by adjusting the stopping point of the system so that the magnetic gap closes and the armature comes to a stop at a point where the slopes of the two curves 26 and 28 are of equal magnitude. This condition appears to be approximated by the practical adjustment just described.

As mentioned, further refinement may be achieved by curving the bearing surface of the lever along a spiral. The expression /2tt depends on the length of the radius of curvature because t is a fraction of the radius. If the radius is long, the system operates on a portion of the leverage curve 26 relatively close to the ordinate axis. If the radius is short, the operation is extended away from the ordinate. In some cases, a relatively long radius may be needed to match the knee of the leverage curve 26 to the knee of the magnetic force curve 28, and when this is done, it may be found necessary, in order to obtain the desired travel, to extend the operation to too steep a part of the leverage curve 26. In such cases, the slope of the leverage curve 26 may be modified, and made to increase less rapidly as it approaches the ordinate by sharpening the curvature of the bearing surface of the lever in the direction toward the pivot. Alternatively, a reduction in the effective radius of curvature may be achieved by providing curved bearing surfaces on both the lever and the armature. In general, however, it is presently believed that satisfactory results may be achieved in most cases with a single curved bearing surface rolling upon an opposed flat surface, and with the curved surface conforming to a circular are having a radius of from about ten to about twenty times the stroke of the armature.

FIGURES 3-5 illustrate a symmetrical actuator according to the invention having two output motions in a balanced configuration. The actuator includes an E-shaped core 30 of laminated construction. The armature 32 is also laminated, and is in the shape of a simple bar. The output motions of the actuator are generated at the opposite respective ends of the armature 32. The construction is identical at both ends. The core 30 is held together by a pair of thin metal straps 34, which are wrapped tightly upon the respective outer legs 36 and 38 of the core. Thin metal cages 40 are fixed to the straps 34 and extend upwardly for loosely retaining the armature 32. Levers (not generally designated) are pivoted adjacent to the inner surfaces of the respective legs 36 and 38, respectively, for transmitting the motion of the armature 32 to a load device or devices. Each lever includes a cylindrically curved bearing plate 42 integrally formed with and extending between a pair of flanges 44, which extend outwardly beyond the legs 36 and 38. A U-shaped plate 46 is fixed to the flanges 44 of each of the levers, and includes arm spring portions 48, which extend inwardly and slightly upwardly. The inner ends of the arms 48 are shaped in the form of upwardly opening books 50 having their bight portions directly in line with the lower outer edges of the bearing plates 42.

The levers are supported upon the upper edges of vertical pivot plates 52, which act as knife edges, and which are supported at their lower ends adjacent to the inner surfaces of the legs 36 and 38 of the core. A support flange 54 is integrally formed with each of the binding straps 34, and extends along the inner face of each of the legs 36 and 38 with its lower edge resting upon the bight surface of the core 30. The flange extends laterally outwardly from the core at both sides thereof. The pivot plate 52 is rigidly secured to the flange 54 at points relatively far from the pivot axis of the lever. This permits to and fro motion of the pivot axis, which is defined by the upper edge of the plate 52 to be accommodated by a relatively slight bending of the plate 52 with a minimum of resistance. The curved bearing plate 42 rests near its outer edge upon the straight upper edge of the pivot plate 52. The hook portions 50 of the U-shaped plate are engaged by downwardly facing edges 60 on the plate 52, which are accurately aligned with the edge upon which the bearing plate 42 rests. The arms 48 are slightly sprung and exert a light biasing force to hold the bearing plate 42 in light pressure engagement upon the edge of the pivot plate 52. Return springs 62 are connected between tabs extending from the outer edges of the U-shaped plates 46 and the opposite respective ends of a base plate 64, which extends along the bottom of the core. Arms 66 are fixed to the flanges 44 for transmitting the rocking motion of the levers to the load.

The actuator is shown in FIGURES 3 and 5 with the armature 32 approximately half way through its travel. When the armature is fully open, it engages the inner upper edge portion of the bearing plates 42. When it is fully closed, it engages the bearing plates 42 along lines relatively close to the pivot axes. In manufacture, the vertical position of the pivot plates is adjusted, as hereinabove explained, to set the spacings between the pivot axes and the lines of contact between the armature and the bearing plates 42 when the armature is in its fully closed position. The objective is to achieve an optimum match between the two curves 26 and 28, and this may be accomplished either by a dynamic testing system or by visual control. In most cases fully adequate matching will be achieved if the pivot axes are adjusted so that the armature 32 completes its travel at a point where the distance between the lines of contact with the bearing plates 42 and the pivot axes is about one-tenth the distance between the lines of contact and the pivot axes when the armature is fully open, that is to say, full travel of the armature should cause the bearing plates 42 to roll along the armature for about of the length of the bearing plates 42.

An actuator actually built in accordance with the embodiment just described and successfully tested has been found to produce an output force that is uniform within about through the full range of its motion. The total travel of the armature was .015". The radius of curvature of the bearing surface was 0.150", and the output rotation was 25 for each of the two levers. It was found that the motion would start about 1.5 milliseconds after the initial application of a constant value energizing voltage to the coil, and that the output motion not only was highly uniform with respect to time, but was completed in about 1.5 milliseconds. In addition, the return motion of the actuator, following de-energization of the coil was also highly uniform with respect to time and at about the same speed as the advance motion. The operating speeds in both directions will, of course, be affected by changes in the strength and rate of the return springs 62, which should be selected in view of the characteristics desired for each utilization.

Itappears to be fully feasible to vary the output motion of the actuator by applying various different energizing voltages to it, and that a fairly uniform proportionality exists over a reasonably wide range between the value of the energizing voltage applied and the point at which the output motion stops. This characteristic when utilized with adequate damping makes the actuator well suited for driving a photographic camera shutter in a mode such that the shutter also serves the function heretofore served by the iris diaphragm. It is also well suited for many other utilizations wherein it is desired controllably to vary the extent of the output motion produced.

FIGURE 6 illustrates an alternative embodiment of the invention, wherein the curved bearing surface is carried on the armature 32', and the bearing plate 42 of the lever is flat. This construction has the advantage that the pivot axis may be positioned close to or at the midpoint of the travel of the armature, and that the curved surface may be produced by die cutting of the laminations of the armature. Its geometry, however, is more difiicult to analyze, but the same general principles apply as hereinabove described. As the armature 32 moves from its open toward its closed position, its lever arm shortens to compensate for the increasing magnetic force so that the output force remains substantially uniform through the entire motion.

What is claimed is:

1. An electromechanical actuator comprising:

(a) a magnetic core,

(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith, and

(c) a lever pivoted adjacent to said gap and abuttingly engaged by said armature for coupling said armature to a load,

(d) the engagement between said armature and said lever being upon a curved surface which is curved in a plane normal to the pivot axis of said lever and convex toward the member engaging it, whereby as said armature advances from its open position toward said core the point of engagement between said armature and said lever moves toward the pivot axis of said lever.

2. An electromechanical actuator comprising:

(a) a magnetic core,

(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith,

(c) a lever abuttingly engaged by said armature for coupling said armature to a load, and

(d) pivot means supporting said lever for rotation about an axis that is freely movable in a direction normal both to said axis and to the direction of travel of said armature.

3. An electromechanical actuator in accordance with claim 2, wherein said pivot means is the edge of a plate, and said plate is rigidly supported at a point spaced far enough from said edge so that fiexure of said plate can accommodate motion of said pivot axis with relatively little resistance.

4. An electromechanical actuator comprising:

(a) a magnetic core,

(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith, and

(c) a lever pivoted adjacent to said gap and abuttingly engaged by said armature for coupling said armature to a load,

(d) the engagement between said armature and said lever being upon a curved surface which is curved in a plane normal to the pivot axis of said lever and convex toward the member engaging it, whereby as said armature advances from its open position toward said core the point of engagement between said armature and said lever moves toward the pivot axis of said lever,

(c) said lever being supported on a pivot which is freely movable in a direction normal to the axis defined by it and normal to the direction of motion of said armature, whereby said lever is free to move to accommodate rolling of said curved surface without sliding upon the surface engaging it.

5. An electromechanical actuator comprising:

(a) a magnetic core,

('b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith, and

(c) a lever pivoted adjacent to said gap for coupling said armature to a load,

(d) said lever including a curved bearing surface abuttingly engaged by said armature, said gearing surface being curved in a plane normal to the pivot axis of said lever and being convex toward said armature, whereby as said armature advances from its open position toward said core the point of engagement between said armature and said lever moves toward the pivot axis of said lever.

6. An electromechanical actuator comprising:

(a) a magnetic core,

-(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith, and

(c) a lever pivoted adjacent to said gap for coupling s a-id armature to a load,

(d) said armature having a curved bearing surface abuttingly engaged by said lever, said bearing surface being curved in a plane normal to the pivot axis of said lever and being convex toward said lever, whereby as said armature advances from its open position toward said core the point of engagement between said armature and said lever moves toward the pivot axis of said lever.

7. An electromechanical actuator comprising:

(a) a magnetic core,

(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction there with,

(c) a lever pivoted adjacent to said gap for coupling said armature to a load,

(d) said lever including a curved bearing surface abuttingly engaged by said armature, said bearing surface being curved in a plane normal to the pivot axis of said lever and being convex toward said armature, whereby as said armature advances from its open position toward said core the point of engagement between said armature and said lever moves toward the pivot axis of said lever, and

(e) pivot means supporting said lever for rotation about an axis that is freely movable in a direction normal both to said axis and to the direction of travel of said armature.

-8. An electromechanical actuator comprising:

(a a magnetic core,

(b) a magnetic armature movable relative to said core and defining a magnetic gap in conjunction therewith,

(c) a lever pivoted adjacent to said gap for coupling said armature to a load,

(d) said armature having a curved bearing surface abuttingly engaged by said lever, said bearing surface being curved in a plane normal to the pivot 7 8 axis of said lever and being convex toward said lever, References Cited whereby as said armature advances from its open UNITED STATES PATENTS position toward sa1d core the polnt of engagement v between said armature and said lever moves toward 2,911,493 11/1959 Weber et 335-428 the pivot axis of said lever, and 5 3,201,541 8/ 1965 Richert 335-276 XR (e) pivot means supporting said lever for rotation I about an axis that is freely movable in a direction GEORGE HARRIS Pnmary Exammer' normal both to said axis and to the direction of travel of said armature. 3 279 C X-R. 

